The present invention pertains generally to solid-state lasers and particularly to tunable color-center lasers (C.C.L.).
In order for a material to be suitable as a continuously tunable solid-state laser, its optical transitions should be homogeneously broadened in both absorption and emission, thus implying a strong electron-phonon coupling of the centers to the lattice. Preferably, the material should have or nearly have a 4-energy level configuration, the four levels consisting of a relaxed ground state, an unrelaxed excited state which is reached through pumping the homogeneously broadened absorption of the center, a relaxed excited state which is thereafter reached by a very rapid (.about.10.sup.-13 sec) nonradiative transition from the unrelaxed excited state, and an unrelaxed ground state which is reached by a radiative transition (.about.10.sup.-8 sec) from the relaxed excited state. Finally, the transition from the unrelaxed ground state back to the initial relaxed ground state is very rapid (.about.10.sup.-13 sec) and nonradiative. The unrelaxed ground state should be at an energy E above the relaxed ground state sufficient to ensure that it not be thermally populated from the ground state, i.e., E&gt;&gt;RT. Furthermore, because of its relatively short lifetime, the unrelaxed ground state would thus be virtually depleted, thereby assuring a population inversion and a low threshold optical pump power. Besides homogeneous broadening and low threshold characteristics, to be most useful as a tunable laser, a material should be easily and inexpensively prepared, have a reasonably high pump efficiency, lase in a desirable wavelength range, and be stable and nonvolatile. An important property, which some color center laser materials do not have is the capability of lasing after periods of storage at room temperature.
Particularly useful ranges of laser tunability are the infrared and near infrared spectral regions, which are important for selecting the specific wavelength of minimum loss for fiber-optic communications, for the molecular spectroscopic analysis of many of the primary and secondary vibrational modes of a wide variety of organic and inorganic molecules, for pollutant detection, and for semiconductor spectroscopic analysis. At present, few acceptable tunable lasers exist for the infrared and near-infrared regions. Organic dyes are moderately effective in the visible region, but fail completely for wavelengths greater than about one micron. Parametric oscillators are useful but are expensive, cumbersome, and often low-powered. The most promising materials for tunable lasing beyond one micron are alkali halides containing homogeneously broadened color centers.
A number of tunable color-center lasers have been reported, e.g., (1) U.S. Pat. No. 3,970,960 issued on July 20, 1976 to Linn F. Mollenauer, (2) B. Fritz et al Laser Effect in KCl with F.sub.A (Li) Centers, in Solid State Communications, 3(3): pp. 61-68 1965, and (3) A Primer on F-center Lasers, in Electro-Optical Systems Design pp. 26-29, September 1978. The color centers which have been made to lase in these systems are the F.sub.A (II) center in several host crystals such as KCl and RbCl, and F.sub.B (II) center in KCl and the F.sub.2.sup.+ center in KCl, NaCl, and KF. The F.sub.A (II) and the F.sub.B (II) centers, useful, are limited to wavelengths between 2.2 and 3.0 microns and have a maximum output power of about 50 mw. Furthermore, it is necessary to use three different crystals in order to cover this spectral range. The F.sub.2.sup.+ center in the presently used crystals, e.g., KC1 and KF, can produce several watts of power very efficiently, but is created in an impractical manner which requires that the crystal first be irradiated with 2 MeV electrons at 77.degree. K., annealed to room temperature or below, and then cooled back to 77.degree. K. The crystal must then remain indefinently at 77.degree. K. in order to retain its color centers and lasing capability. Unfortunately, crystals containing F.sub.2.sup.+ centers produced in this manner exhibit a long-term optical fatigue that causes the laser capability of the material to disappear. These crystals then require an impractical continued reprocessing with a 1 to 2 MeV-electron accelerator.